My husband works with his ex girlfriend, should I be concerned?
Has your husband only recently started to work with his ex-girlfriend? Or have you only recently found out that they have worked together for some time?
I'm assuming one of these is true since, if they were in a long-term coworker relationship quite platonically and normally, and you already knew about it, then you wouldn't have any reason to ask the question.
So my guess is that something has recently happened to raise your suspicions.
If it's one of the two scenarios I suggested, then your first port of call is to be straightforward and talk to your husband about it. Ask him how he gets on with the ex and how he feels about having to work with her. Or if they have already been coworkers for some time, ask him why he didn't mention it to you.
And take it from there.
The odds are that it's just a normal coworker relationship. Most people leave their exes for good reasons - and rarely want to get back together with them. That's even more so after they are married. And he married you, not her.